If the funds in the hands of any county treasurer at any time arising under §§ 21-403, 21-404, 21-405, 21-408 or 21-409, or in any other manner, shall be greater than is necessary to pay the annual installments of principal and interest, or the annual cost of maintenance of the drainage works, or both, such surplus shall be held by the county treasurer for future disbursements for other purposes as herein provided or subject to the order of the county board of drainage commissioners of the county in which the petition was filed.